1. Field of the Invention
The invention relates to real-time image interpolations particularly to apparatus and methods for interpolating between lines of an interlaced video signal to produce a progressive video picture.
2. Description of the Prior Art
The video line doubling for the conversion from an interlaced signal to a progressive one is an interesting subject for upgrading picture quality. The technical literature and the patents related to this subject are abundant. The existing techniques can be divided in the four following classes: Purely Spatial Adaptation (PSA); Vertical Temporal Adaptation (VTA); Fixed Vertical Temporal Filtering (FVTF); and Motion Based Adaptation (MBA).
In the first class namely PSA (see the article by T. Doyle & M. Looymans "Progressive Scan Conversion using Edge Information" in Signal Processing of HDTV, II L. Chiariglione (ed.) Elsevia Science Publishers, BU, 1990 pp 711-721, and U.S. Pat. Nos. 5,019,903 or 5,347,599, the interpolation is purely spatial and is based on some edge orientation detection. The detection, in turn, is based directly on various oriented pixel differences. There are some drawbacks to this technique. The given edge detectors are not very reliable for a noisy or high frequency signal. This phenomenon produces some temporal flickers since the adaptation at a given edge can be different from field to field. Moreover the spatial interpolation yields some resolution loss in the still part of the picture.
In the VTA class (see published GB Pat. Appln. 2,184,628-A), the interpolation results from a purely vertical interpolation or a purely temporal interpolation. The adaptation for these two modes of interpolation is driven by a motion detector. The shortcomings are: the resolution loss for moving parts; the staircase effect on diagonal edges; and the unnatural signal yielded from median like vertical interpolation filters.
The FVTF class was first suggested in the article by T. Fukinuki, Y. Hirano "Extended Definition TV Fully Compatible with Existing Standards", IEEE Trans on Communications, Vol Com-32, No. 8 August 1984 pp 948-953. Fukinuki and Hirano teach the use of a halfband vertical temporal filter for the down conversion from progressive to interlaced format. Conversely, the same filter with appropriate gain can be used for the up conversion (see published EP Pat. Appln. 0,266,079-A2). However, if the interlaced video is not prefiltered, this conversion technique introduces some artifacts in the interpolated video output: staircase effect on moving diagonal edges, vertical resolution loss in the still picture parts, and line flickers of vertically moving horizontal edges.
Finally, the interpolation technique using estimated motion (MBA) offers a promising alternative. However, the MBA is generally costly and the used movement model is not always reliable.